Support simple variable assignment

boot/stage3.elna

@@ -4,12 +4,8 @@
 
 # Stage3 compiler.
 #
-# - Procedures without none or one argument.
-# - Goto statements.
-# - Character and integer literals.
-# - Passing local variables to procedures.
-# - Local variables should have the format: v00,
-#   where 00 is its offset from the sp register.
+# - Procedures with multiple arguments.
+# - Character literals with and without escaping.
 
 .section .rodata